1. Field of the Invention
The present invention relates to an electronic element for protecting a semiconductor integrated circuit (IC), and more particularly, to an electronic element used in a circuit for protecting and switching specific circuits by the use of reversible or irreversible field emission of cold cathodes. It also relates to an electronic element used as an active element with triode characteristics which utilizes cold cathode emission.
2. Description of the Prior Art
In recent years, the dimension of semiconductor devices formed in an IC has become about 1 .mu.m, and about one million semiconductor devices are integrated on an IC chip.
There has been a great demand for various functions in semiconductor devices to be integrated in an IC. Examples of the semiconductor devices include flip-flop circuits, NAND gates composed of complementary MOS transistors, which are used as logical circuits, dynamic random access memories (DRAMs), static random access memories (SRAMs), read only memories (ROMs), electrically programmable random access memories (EPROMs), electrically erasable programmable random access memories (EEPROMs), analog circuits, bipolar transistors, and fuses and anti-fuses capable of switching circuits.
In production of DRAMs or SRAMs which are large-scale integrated circuits, there is a high probability that defects may occur in these integrated circuits during the production steps thereof. To eliminate this problem, it is useful to replace a faulty memory with a spare memory previously provided by using a fuse or an anti-fuse. In the case of a fuse, all of the circuits are connected to each other, and then a fuse connected to a faulty circuit is eliminated by applying a high voltage or by irradiating with a laser beam. In the case of an anti-fuse, all of the circuits are not connected to each other, and then only the portion where an electrical connection is desired is applied with a high voltage or a laser.
In view of the above function of a fuse or an anti-fuse, the range of applications thereof is wide. For example, when a fuse or an anti-fuse is provided in each memory element, required data can be written in a predetermined memory element at any time. When a fuse or an anti-fuse is provided in a logical circuit, the logical circuit can be switched even after the fabrication thereof.
Electronic circuits are integrated on a semiconductor chip on a large scale, so that the construction thereof has been complicated. Various apparatus using an IC has rapidly been produced one after another, accompanied by the development of an IC. To shorten the period required for producing an IC, attempts have been made so that various kinds of circuits to be integrated on a semiconductor chip are previously constructed so as to change the combination of the circuits or switch the circuits after the integration. For example, a circuit formed in a semiconductor chip comprising an EPROM and an EEPROM as a microcomputer circuit is realized for the above purpose.
A conventional example using the above-mentioned fuse will be described. An oxide film and polycrystalline silicon (i.e., polysilicon) are deposited on a semiconductor substrate in this order. Impurities are diffused in the polysilicon to form a polysilicon island having an appropriate dimension. Predetermined two circuits are connected to each other through this island. When it is not necessary to connect these two circuits to each other, an excess amount of voltage is applied to both ends of the polysilicon which thereby fuses to eliminate the connection.
A conventional example using the above-mentioned anti-fuse will be described. A diffused layer is formed on a semiconductor substrate. On the diffused layer, a silicon nitride film and a metal layer are formed in this order. This metal layer is connected to the diffused layer through the silicon nitride film. Two predetermined circuits are electrically connected to the metal layer and the diffused layer, respectively. Under this condition, since the silicon nitride film is an insulating film, the metal layer is electrically insulated from the diffused layer. When it is necessary to electrically connect these two circuits, an excess amount of voltage is applied to the silicon nitride film to destroy it, resulting in an electrical connection between the metal layer and the diffused layer.
A semiconductor memory is selectively used in accordance with the types and applications of data. In general, an ROM is employed for repeatedly using the same data. When data to be written is not determined or when it is necessary to change data which has been written, various read only memories such as EPROMs and EEPROMs are used, which are UV-light or electrically erasable and electrically programmable. A DRAM is also employed for repeatedly writing data at suitable intervals so as to maintain written data. In a DRAM, written data is not maintained when a power source is turned off, so that a DRAM is suitable for frequently rewriting data. In an SRAM, written data can be maintained by flowing a smaller amount of current compared with that used for a writing operation. In the programmable memories such as EPROMs and EEPROMs, written data can be maintained even after a power source is turned off, because the data has been physically written therein.
Minute semiconductor devices described above are likely to be damaged by noise or surge entering from outside. To prevent this problem, a semiconductor device is provided with a protective circuit at its external portion. This protective circuit comprises a protective element for preventing an excess voltage such as spike noise and surge from being applied to circuits formed inside the semiconductor device. Moreover, signal lines are interrupted by using a power source wiring or a ground wiring, or a resistor constituted by a diffused layer or a PN junction is connected to an input/output terminal of an IC, thereby preventing surge and excess current from flowing into the IC.
An example will be described with reference to FIG. 17, in which a diffused layer (PN junction capacitance) is used for the purpose of protecting internal circuits when an excess voltage is applied to an external circuit which performs an input/output operation.
A power source Vdd is connected to the source of a transistor T1 and to a diode D1. The other end of the diode D1 is connected to an input Vin which is connected to the gate of the transistor T1. An output Vout is obtained from the drain of the transistor T1. The drain of the transistor T1 is grounded through a resistor R1.
The diode D1 functions as a protective diode by which the voltage of the input Vin does not become higher than that of the power source Vdd. When the voltage of the input Vin is lower than that of the power source Vdd, the protective diode D1 is reverse-biased, thereby preventing a current from flowing therethrough. When the voltage of the input Vin becomes higher than that of the power source Vdd, the diode D1 is forward-biased.
In a protective circuit using the above-mentioned diffused layer, there are two problems described below.
To protect circuits from unexpected spike noise and surge, a diffused layer having a certain area is required. However, it is difficult to reduce the size of the diffused layer along with the miniaturization of a semiconductor device.
Since a diffused layer prevents a voltage from rapidly changing due to surge or the like by using the junction capacitance and resistance of the diffused layer, or a protective diode, the transmission rate of input signals is delayed because of these capacitance and resistance.
When polysilicon is used as a fuse, the polysilicon is connected to one part of a wiring in a circuit. The resistance of the wiring connected to the fuse is high, so that the operation rate of the entire circuit is decreased. This problem can be solved by making the width of the fuse larger to decrease the resistance of the wiring. However, the application of a high voltage is required for cutting down this fuse, which cannot be realized.
When an anti-fuse is used, that is, when a silicon nitride film is formed on a diffused layer overlying a silicon substrate, and a metal layer is disposed on the silicon nitride film, the anti-fuse-formed area becomes large, thereby decreasing the degree of integration of semiconductor devices in a chip. To improve the degree of integration, the anti-fuse can be formed in the later step such as a wiring step. A structure of such a semiconductor device or a method for forming the same have not yet been proposed.
Cold cathode emission from electrodes is described, for example, in "Vacuum Microelectronics" (J. Itoh, Applied Physics, Vol. 59, No. 2 (1990) pp. 164-169). This literature discloses the formation of electrodes on various kinds of substrates by using a minute processing technique. An element obtained by this technique should be operated in a vacuum atmosphere. When this element is used in the air, the electrodes can readily be oxidized or damaged, so that the cold cathode emission properties thereof are rapidly degraded.